1] Get your video clip and open it in quicktime pro. Edit it to the right length and export it as 'movie to image'. This will create lots of single frame images so put them in a new folder. You can decide the frame rate - say 5 or 10 per sec
2] Open the first frame in photoshop (I use CS2 on a mac) and select the animation sub window. Open the second picture, select all, copy and paste onto the first picture. This creates a new layer on picture 1. Repeat this for all the frames - i.e. create new layers of all the frames as layers in picture one. In the animation window on the right there is a menu - select 'make frames from layers'. You can select the duration of the frames but usually leave at '0 sec'. 'Save for web' from the file menu creating a gif file. It actually makes 2 files - the gif is in a folder called 'images'
3] Open your postimage account and upload the gif file which is in the 'images' folder. Make sure it is less than 1MB
4] Open your blogger editor and paste the embed code from postimage